<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box {
        width: 100px;
        height: 100px;
        background: red;
        margin: 10px;
    }

    .box3 {
        width: 100px;
        height: 100px;
        background: blue;
        margin: 10px;
    }

    .box:hover+.box3 {
        opacity: 0;
    }
</style>

<body>
    <div class="box"></div>
    <div class="box"></div>
    <!-- <div></div> -->
    <div class="box3"></div>
</body>

</html>